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Rheological properties of styrene–butadiene rubber filled with electron beam modified surface treated dual phase fillers
ARTICLE IN PRESS
Radiation Physics and Chemistry 69 (2004) 91–98
Rheological properties of styrene–butadiene rubber filled with electron beam modified surface treated dual phase fillers A.M. Shanmugharaj, Anil K. Bhowmick* Rubber Technology Centre, Indian Institute of Technology, Kharagpur-721302 India Received 16 September 2002; accepted 21 March 2003
Abstract The rheological properties of styrene–butadiene rubber (SBR) loaded with dual phase filler were measured using Monsanto Processability Tester (MPT) at three different temperatures (100 C, 110 C and 130 C) and four different shear rates (61.3, 306.3, 613, and 1004.5 s1). The effect of electron beam modification of dual phase filler in absence and presence of trimethylol propane triacrylate (TMPTA) or triethoxysilylpropyltetrasulphide (Si-69) on melt flow properties of SBR was also studied. The viscosity of all the systems decreases with shear rate indicating their pseudoplastic or shear thinning nature. The higher shear viscosity for the SBR loaded with the electron beam modified filler is explained in terms of variation in structure of the filler upon electron beam irradiation. Die swell of the modified filler loaded SBR is slightly higher than that of the unmodified filler loaded rubber, which is explained by calculating normal stress difference for the systems. Activation energy of the modified filler loaded SBR systems is also slightly higher than that of the control filler loaded SBR system. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Styrene–butadiene rubber (SBR); Electron beam; Filler; Rheology; Die swell; Activation energy
1. Introduction The knowledge of rheological properties of elastomers is of considerable importance in predicting and comprehending their processing characteristics. Both the viscous and elastic properties of elastomer can be analyzed and correlated with the flow behavior. The viscous flow is related to the output rate, whereas the elastic behavior corresponds to the dimensional stability. Einhorn and Turetzky (1964) have shown the use of capillary rheometer to characterize elastomeric flow of carbon black filled styrene–butadiene rubber (SBR) systems. The processing behavior of carbon black filled elastomers and their blends have also been well *Corresponding author. Tel.: +91-3222-282037; fax: +913222-277190. E-mail address: [email protected] (A.K. Bhowmick).
explained in various literatures (Leblanc, 2002; Nakijama and Collins, 1975; Bhaumik et al., 1990; Kumar et al., 2002; Osanaiye et al., 1995). Cotton, 1979 has studied the post-extrusion relaxation phenomenon over a wide spectrum of carbon blacks using a capillary rheometer. Recently, the processing behavior of finely divided silica in combination with bifunctional silane has been reported (Schaal and Coran, 2000; Schaal et al., 2000). In these references, the rheological behavior of silica filled SBR has been explained as a function of filler type, filler loading, temperature, storage time and storage temperature. Recently, Cabot Corporation has designed new filler by co-fuming technology of carbon and silica, which is termed as carbon–silica dual phase filler having higher polymer–filler interaction and lower filler–filler interaction (Murphy et al., 1998). Even though, this new filler has lower amount of silica phase, its reinforcement behavior is close to silica. From our earlier communications (Shanmugharaj and Bhowmick, 2002a; Shanmugharaj
0969-806X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0969-806X(03)00331-1
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modified dual phase fillers in presence and absence of various sensitizers.
and Bhowmick, 2003; Shanmugharaj et al., 2002b), we have concluded that modification of carbon–silica dual phase filler by electron beam treatment results in surface oxidation and structure development that enhances certain properties like modulus, tensile strength and elongation at break. In this paper, we discuss the rheological behavior of SBR loaded with electron beam
2. Experimental 2.1. Materials Polymer: Styrene–butadiene rubber (SBR-1502, styrene content-23.5%, ML1+4 100 C, 51) was supplied by Synthetic and Chemicals Ltd, Barielley, India. Filler: Dual phase filler (CSDPF-A, Silica content 4.7%, N2 Surface area 167 m2/g) was supplied by Cabot Corporation, Billerica, USA. Trimethylol propane triacrylate: TMPTA was supplied by UCB Chemicals, Belgium. Silane coupling agent: Triethoxysilylpropyltetrasulphide (Si-69) was supplied by Degussa Company, Germany.
Table 1 Specification of the electron beam accelerator Energy range Beam power over the whole energy range Beam energy spread Average current (E1.5 MeV) Adjusting limits for current Accelerating voltage frequency Duration Repetition rate Pulse rate Maximum Minimum Power supply (PS) voltage PS voltage frequency Consumption of power (total)
0.5–2.0 MeV 20 kW 710% 15 mA 0–30 mA 100–120 MHz 400–700 ms 2–50 Hz
2.2. Filler modification The detailed procedures for the modification of filler using electron beam accelerator were discussed in our earlier communications (Shanmugharaj and Bhowmick, 2002a; Shanmugharaj et al., 2002b). The specifications of the electron beam accelerator are reported in Table 1 (Ray and Bhowmick, 2001).
900 mA 900 mA 3 380/200 V 50 Hz 150 kW
2.3. Preparation of filled SBR Table 2 Compounding formulation Sl. No
Ingredients
Loading (phr)
Mixing time (min)
1 2 3 4 5
SBR 1502 Filler ZnO Stearic acid Polymerized trimethyl quinoline
100.0 20.0 5.0 1.5 1.0
1 4 1 1 1
The compounds according to the formulation (Table 2) were mixed in a Brabender Plasticorder (model PLE 330, capacity 65 ml) at a rotor speed of 60 rpm and at 80 C. In all cases, mixing time was kept at 8 min. 2.4. Designation of filled SBR SBR masterbatch was designated as SBx/y/z, where S and B represent SBR and dual phase filler respectively, and the suffixes x; y and z represent monomer or silane, radiation dose (in kGy) and filler loading, respectively. The sample designations are tabulated in Table 3.
This filler master batch was passed in two-roll mill for 2 min at 30 C and used for rheological measurements.
Table 3 Sample designation for gum and filled SBR Sl. No
Sample designation
Filler
Modifier
% of modifiers
Radiation dose (kGy)
Filler loading (phr)
1 2 3 4 5 6 7 8
S0 (gum) SB0/0/20 SB0/100/20 SB0/200/20 SB1T/100/20 SB3T/100/20 SB1S/100/20 SB3S/100/20
— CSDPF-A CSDPF-A CSDPF-A CSDPF-A CSDPF-A CSDPF-A CSDPF-A
— — — — TMPTA TMPTA Si-69 Si-69
— — — — 1 3 1 3
— — — 100 200 100 100 100
— 20 20 20 20 20 20 20
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2.5. Measurement of rheological properties The melt flow properties of the gum SBR (without filler) and the unmodified and the electron beam modified filler loaded SBR were measured by means of a Monsanto Processibility Tester (MPT) (barrel radius, 9.53 mm) which is a fully automated capillary viscometer. The entire barrel and the capillary assembly are electrically heated with a microprocessor-based temperature control. The capillary used had a length to diameter ratio equal to 30 (length 30.00 mm; diameter 1.00 mm). The compound entrance angles of capillary were 45 and 60 , which are known to minimize the pressure drop at entrance. Therefore, the Bagley correction can be assumed to be negligible and the apparent shear stress can be taken as equal to the true shear stress. The preheated time for each sample was 5 min. The extrusion studies were carried out at three different temperatures (100 C, 110 C and 130 C) and at four different shear rates (61.3, 306.3, 613 and 1004.5 s1). The rate of shear variation was achieved by changing the speed of the plunger automatically. The pressure at the entrance of the capillary was recorded automatically with the help of pressure transducer. The apparent shear stress (tapp ), apparent shear rate (gapp ) and apparent shear viscosity (Zapp ) were calculated using the following equations (Brydson, 1981): tapp ¼
dc DP ; 4lc
ð1Þ
g’ app ¼
32Q ; pdc3
ð2Þ
Zapp ¼
tapp ; g’ app
ð3Þ
where DP is the pressure drop across the length of the capillary, dc and lc are diameter and length of capillary, respectively; Q; the volumetric flow rate of the material. The flow behavior index, n and consistency index, k were calculated by using the Power Law model tapp ¼ k’gapp
where C¼
3n0 þ 1 ; 4ðn0 þ 1Þ
ð8Þ
where a is the extrudate swell and n0 is equal to n, the flow behavior index determined from the slope of log (apparent shear viscosity) (Zapp ) vs. log (shear rate) (’gapp ) plots, following Eq. (6): 2.7. The principle normal stress differences The principle normal stress differences, sE ; in an elastic body, is calculated by using the following general equation (Vinogradov and Ya Malkin, 1979): sE ¼
ð2 þ gm Þgm ð2tÞ; 2ð1 þ gm Þ
ð9Þ
where t is the shear stress, gm the maximum recoverable deformation which is a complex function of filler loading, temperature and the nature of the fluid. 2.8. Activation energy of melt flow Activation energy of viscous flow was derived from the Arrhenious–Frenkel–Eyring (Vinogradov and Ya Malkin, 1979), which is valid for the power law fluids, as follows: Zapp ¼ BeEg =RT ;
ð10Þ
where, Eg is the activation energy of the flow at a particular shear rate, R; the gas constant, T; the temperature in Kelvin, B, the pre-exponential component and Zr ; the viscosity in Pa s at that shear rate. For the different systems, from the slope of the linear plot of log viscosity vs. reciprocal of temperature (1/T), the value of Eg =R was obtained from which the activation energy was calculated.
3. Results and discussion ð5Þ 3.1. Rheological behavior of electron beam modified filler loaded SBR
Logarithmic form for Eq. (5) may be written as log Zapp ¼ log k þ ðn 1Þlog g’ app :
ð7Þ
ð4Þ
by definition Zapp ¼ tapp =’gapp ; therefore Zapp ¼ k’gn1 app :
et al., 1992a): rffiffiffiffiffiffiffi 1 4 gm ¼ ða þ 2a2 3Þ; 2C
93
ð6Þ
The values n and k; were calculated for the initial linear region observed at lower shear rate. 2.6. Maximum recoverable deformation Maximum recoverable deformation, gm was calculated using the following equations (Kumar
Fig. 1 shows the apparent shear viscosity vs. shear rate plots for the gum and the unmodified and the electron beam modified dual phase filler loaded styrene– butadiene rubber at three different temperatures (100 C, 110 C and 130 C). The viscosity decreases with the shear rate showing the pseudoplastic or shear thinning nature of all the systems studied. Both the gum SBR and its filled compounds follow the power law model. The
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Fig. 1. Log (Apparent shear viscosity) vs. log (Apparent shear rate) plots of gum and electron beam modified filler loaded SBR at (a) 100 C (b) 110 C (c) 130 C.
Fig. 2. Log (Apparent shear viscosity) vs. log (Apparent shear rate) plots of TMPTA and silane modified filler loaded SBR at (a) 100 C (b) 110 C (c) 130 C.
viscosity of the gum SBR is lower than that of the filled systems at all the temperatures studied. As compared to the unmodified system (SB0/0/20), the modified filler loaded SBRs (SB0/100/20 and SB0/200/20) show slightly higher viscosity especially at lower shear rate region (Fig. 1). From TEM results (Shanmugharaj and Bhowmick, 2002a), it has been concluded that electron beam modification of dual phase filler results in variation of primary and secondary structures. It has been previously reported that anisometry of the filler aggregates and occluded rubber, which are associated with the structure of the filler may play an important role in compound viscosity (Patat et al., 1966; McCabe, 1965). The occlusion of the rubber within the aggregates enhances the effective hydrodynamic volume of the filler. In the case of modified filler loaded SBRs (SB0/100/20 and SB0/200/20), higher polymer occlusion takes place within the aggregates, which in turn enhances the effective hydrodynamic volume of the filler, giving rise to higher viscosity. Fig. 2 compares the viscosity vs. shear rate plots of TMPTA and silane modified dual
phase filler loaded SBR compounds (SB1T/100/20, SB3T/100/20, SB1S/100/20 and SB3S/100/20). The plots suggest that there is no significant variation in apparent viscosity for these modified filler loaded systems. In our earlier communications (Shanmugharaj and Bhowmick, 2002a) we have concluded that TMPTA or silane occupies the pores of the dual phase filler that significantly affects the rubber occlusion when this fillers are loaded in rubber. The consistency index, k and flow behavior index, n for this modified filler loaded SBR vulcanizates are included in Table 4. It is quite obvious that consistency index, k is more for the filled elastomer compared to the gum rubber. However, the consistency index, k is still higher for SB0/100/20 and SB0/200/20 compared to the control (SB0/0/20) showing the increase in resistance to flow. However, modification of the filler with TMPTA or with silane slightly improves the index value indicating the higher resistance to flow for this modified filler loaded SBR systems. The higher rubber occlusion due to its high structure and higher polymer–filler
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Table 4 Flow behavior index, n and consistency index k 104 (Pa sn) for SBR loaded with modified fillers Sl. No
Samples
Temperature 100 ( C)
1 2 3 4 5 6 7 8
S0 SB0/0/20 SB0/100/20 SB0/200/20 SB1T/100/20 SB3T/100/20 SB1S/100/20 SB3S/100/20
110 ( C)
130 ( C)
n
k ( 104 Pa sn)
n
k ( 104 Pa sn)
N
k ( 104 Pa sn)
0.40 0.38 0.38 0.35 0.39 0.37 0.35 0.35
37.33 48.08 48.98 66.83 48.53 52.36 58.62 60.30
0.41 0.44 0.43 0.41 0.39 0.42 0.42 0.50
30.40 33.82 42.39 49.85 42.98 48.12 42.10 48.10
0.42 0.49 0.51 0.42 0.42 0.41 0.41 0.43
27.30 30.43 34.91 35.89 36.39 39.90 33.42 39.80
Fig. 3. Variation of die swell with shear rate for gum and electron beam modified filler loaded SBR.
interactions due to chemical interaction in modified filler loaded systems (SB0/100/20, SB0/200/20, SB1T/100/20, SB3T/100/20, SB1S/100/20 and SB3S/100/20) results in higher consistency index for this modified filler loaded SBRs. Consistency index, k decreases with increase in temperature resulting in decrease in resistance to flow for the gum and the filled systems. The variation in flow behavior index, n; shows the pseudoplastic behavior of these SBR systems. 3.2. Elastic effects in melt flow 3.2.1. Effect of modified fillers on die swell of SBR Fig. 3 shows the variation of die swell with shear rate for the gum and the unmodified and the electron beam modified dual phase filler loaded SBR. Die swell remains almost constant upto a shear rate of about 306.3 s1 and then decreases at higher shear rate for the modified filler loaded systems. Die swell is low for the filled SBR compared to the gum rubber (S0 ), which is attributed to the polymer–filler interactions, reduction in polymer content per unit volume of the compound, decrease in elastic nature and development of higher viscosities. The
Fig. 4. Linear plots of log (normal stress difference) vs. log (shear stress) for gum and electron beam modified filler loaded SBR.
variation in die swell is almost same for all the filler loaded SBR at lower shear rate. However, at higher shear rate, the values of die swell are entirely different. SB0/100/20 and SB0/200/20 show higher values at higher shear rate (1004.5 s1) compared to SB0/0/20. In order to understand the elastic effects of this modified filler loaded SBR systems, principle normal stress difference is calculated at different shear rates using Eqs. (6)–(9). The logarithmic value of this normal stress difference when plotted against log (shear stress) gives a straight line as per the following equation (Kumar et al., 1992a) (Fig. 4): sE ¼ IðtÞm ;
ð11Þ
Log sE ¼ logðIÞ þ m logðtÞ:
ð12Þ
The slope and intercept values calculated by using the best-fit lines at two temperatures (110 C and 130 C) are included in Table 5. The slope value increases and the intercept value marginally decreases at both the temperatures (110 and 130 C) for electron beam modified dual phase filler loaded SBRs (SB0/100/20 and
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SB0/200/20) as compared to the control system (SB0/0/20). This is attributed to the decrease in elastic effect due to higher rubber occlusion, which enhances the viscous nature of the filled elastomer. The slope and intercept values of the double logarithmic plot of principle normal stress difference vs. shear stress (Table 5) confirms the above fact. Fig. 5 shows the plot of die swell vs. shear rate for TMPTA or silane modified dual phase filler loaded SBR. The die swell is almost same upto the shear rate of 306.3 s1 for TMPTA (SB1T/100/20 and SB3T/100/20) or silane (SB1S/100/20 and SB3S/100/20) modified filler loaded SBRs. However, it decreases with increase in silane level at higher shear rate (1004.5 s1). 3.2.2. Effect of temperature on die swell An increase in temperature reduces die swell of both the rubber and the filled systems (Fig. 6). The reduction in die swell is more in the case of gum than the filled systems. This can be well understood from the higher negative slope value (0.19) in the linear plot of die swell with temperature (Fig. 6a). The higher intercept Table 5 Slope and intercept values of gum and filled SBR calculated using Eq. (12) Sl. No Samples
Slope (m)
Intercept [log (I)]
at 110 C at 130 C at 110 C at 130 C 1 2 3 4 5 6 7 8
S0 SB0/0/20 SB0/100/20 SB0/200/20 SB1T/100/20 SB3T/100/20 SB1S/100/20 SB3S/100/20
0.95 0.97 1.03 1.05 0.98 1.05 1.04 1.05
0.89 0.91 0.94 0.97 0.92 0.93 0.91 0.93
2.00 1.96 1.84 1.78 1.85 1.81 1.86 1.81
2.15 2.10 2.07 1.93 2.07 2.04 2.12 2.00
Fig. 5. Variation of die swell with shear rate for TMPTA and silane modified filler loaded SBR.
Fig. 6. Variation of die swell with temperature for (a) gum and electron beam modified dual phase filler loaded SBR. (b) TMPTA and silane modified dual phase filler loaded SBR. Table 6 Slope and intercept values of the linear plot of die swell vs. temperature Sl. No
Samples
Slope
Intercept
1 2 3 4 5 6 7 8
S0 SB0/0/20 SB0/100/20 SB0/2200/20 SB1T/100/20 SB3T/100/20 SB1S/100/20 SB3S/100/20
0.19 0.17 0.11 0.08 0.06 0.12 0.10 0.12
81.20 67.90 64.50 62.50 58.06 65.86 64.00 65.66
value (81.20) for the gum rubber (Table 6) is attributed to higher rubber entanglements that store the elastic energy. The increase in temperature increases the mobility of the polymer chain, which reduces the capacity of polymer molecules to store the elastic energy and hence there is a reduction in die swell values (Kumar et al., 1992b). However, in the case of the filled rubber (SB0/0/20), the negative slope decreases to 0.17 and also the intercept value to 67.2. Addition of filler in rubber reduces the polymer content per unit volume, thereby decreasing the elastic nature. This is responsible for the lower intercept value for the filled elastomer. The lower negative slope for the filled elastomers is attributed to higher rubber occlusion in fillers that restricts the mobility of polymer chains on increasing the
ARTICLE IN PRESS A. M. Shanmugharaj, A.K. Bhowmick / Radiation Physics and Chemistry 69 (2004) 91–98
temperature. On incorporating the modified fillers in SBR (SB0/100/20 and SB0/200/20), the intercept and the negative slope values still decrease. This is also due to occluded rubber in the filler aggregates and rubber–filler interaction, which balance the effect of temperature. Presence of TMPTA or silane in the modified fillers reduces the effect of temperature on die swell for SBR composites (SB1T/100/20 and SB1S/100/20) compared to SB0/100/20 (Fig. 6b). This can be well documented from the slope and the intercept values (Table 6). However, on increasing the TMPTA or silane level in the filler, both the negative slope value and intercept value increases. Even though, rubber–filler interaction is more in SB3T/100/20 and in SB3S/100/20, rubber occlusion will be less due to the presence of TMPTA or silane in pores of filler. Hence, polymer per unit volume will be more in SB3T/100/20 and in SB3S/100/20 compared to SB1T/100/20 and SB1S/100/20 resulting in polymer flow with temperature.
3.3. Activation energy of melts flow Activation energy of the flow process has been calculated from the slope of the plot of log (shear viscosity) vs.1/T as given by Eq. (10) (Vinogradov and Ya Malkin, 1979). Fig. 7 shows the dependence of activation energy on the rate of shear for electron beam modified filler loaded SBR Activation energy of melt flow for the gum rubber (S0 ) is almost same for over the range of shear rates
97
investigated (Fig. 7a). SB0/0/20 and SB0/100/20 show almost constant value at lower shear rate (upto 306.3 s1) and decreases with further increase in shear rate. However, activation energy drastically reduces for SB0/200/20 even at lower shear rate (at 306.3 s1). This decrease in activation energy with increasing shear rate may be due to: (i) orientation of the molecular segments in the direction of applied stress and (ii) increased wall slip (Kumar et al., 1992b). Electron beam modification of filler results in variation of primary and secondary structures, which increases with radiation dose. The fractal dimensions as observed from TEM studies (Shanmugharaj and Bhowmick, 2002a) are 1.26 for the control filler (B0/0) and 1.36 for the electron beam modified filler (B0/100), respectively. This variation in structure results in more rubber occlusion that enhances the viscosity at lower shear rate, which is responsible for higher activation energy. However, at higher shear rates, breakdown of structures results in release of occluded rubber leading to wall slippage that decreases the activation energy of melt flow and this effect is more pronounced in SB0/200/20. The Eg value is low for SB0/0/20 and SB0/100/20, whereas it is high for SB0/200/20 compared to the gum rubber (S0 ) at higher shear rate (1004.5 s1). The lower value of Eg for SB0/0/20 and SB0/100/20 is attributed to the reduction in matrix viscosity due to decrease of rubber and broadening of molecular weight distribution (MWD) (Kumar et al., 1992a). The break down of structure at this shear rate releases rubber molecules with lower molecular weight thereby it increasing the molecular weight distribution, which decreases the matrix viscosity. Even though, structure breakdown is prominent in SB0/200/20, there may be significant amount of structure that enhances the matrix viscosity, which is responsible for the higher activation energy for SB0/200/20 compared to the gum (S0 ) at higher shear rate. Fig. 7b shows the variation of activation energy of melt flow with shear rate for TMPTA and silane modified dual phase filler loaded SBR. Activation energy decreases sharply with shear rate for all the modified filler loaded SBR systems. However, with increase in TMPTA or silane level, the activation energy of melt flow at lower shear rate region (61.3 s1) increases, which is attributed to the higher surface activity of the TMPTA and silane modified filler with the rubber matrix. However, at higher shear rate (1004.5 s1) the activation energy decreases with silane level due to dispersion of filler that reduces the matrix viscosity.
4. Conclusions Fig. 7. Variation of activation energy with shear rate for (a) gum and electron beam modified dual phase filler loaded SBR. (b) TMPTA and silane modified dual phase filler loaded SBR.
In the present investigation, the melt flow properties of the gum and the unmodified and the electron beam
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modified dual phase filler loaded SBR were measured and the following conclusions were drawn: 1. Apparent shear viscosity decreases with increasing shear rate for the gum and the unmodified and the electron beam modified dual phase filler loaded SBRs showing the pseudoplastic or shear thinning nature of the system. 2. The higher shear viscosity at all temperatures (100 C, 110 C and 130 C) for the modified filler loaded SBRs is attributed to higher rubber occlusion due to its high structure compared to the control dual phase filler loaded SBR. 3. Die Swell is almost same with increase in shear rate upto 306.3 s1 beyond which it decreases for the filler loaded SBR systems. This is explained by calculating normal stress difference for all the systems. 4. The activation energy of the melt flow process for filled SBR compounds decreases with increase in shear rate due to orientation of the molecular segments in the direction of applied stress and increased wall slippage.
Acknowledgements The authors are grateful to Dr. M. J. Wang, Cabot Corporation USA for providing the dual phase fillers. Dr. A. B. Majali & Dr. S. Sabharwal of Bhabha Atomic Research Centre, Mumbai for assistance in the electron beam work and Dr. V. K. Tikku of NICCO Corporation Ltd., Calcutta for his timely help.
References Bhaumik, T.K., De, P.P., Bhowmick, A.K., Gupta, B.R., 1990. Rheological behavior of blends of ethylene-propylene-diene and bromobutyl rubbers. Indian J. Technol. 28, 33–38. Brydson, J.A., 1981. Flow Properties of Polymer Melts, 2nd Edition. George Godwin Limited, London, pp. 1–17. Cotton, G.R., 1979. Influence of carbon black on processability of rubber stocks II. Extrusion shrinkage. Rubber Chem. Technol. 52, 187–198. Einhorn, S.C., Turetzky, S.B., 1964. Rheological properties of SBR polymers by capillary extrusion. J. Appl. Polym. Sci. 8, 1257–1273.
Kumar, N.R., Gupta, B.R., Bhowmick, A.K., 1992a. Viscous and elastic effects during processing of gum and filled natural rubber. Kautschuk Gummi Kunststoffe 45, 531–536. Kumar, N.R., Gupta, B.R., Bhowmick, A.K., 1992b. Rheological behavior of acrylic and epoxidised natural rubbers. J. Polym. Eng. 12, 105–124. Kumar, B., De, S.K., Bhowmick, A.K., De, P.P., 2002. Rheological behavior of brominated isobutylene-co-paramethyl styrene (BIMS): Effect of fillers, oil and blending with EPDM. Polym. Eng. Sci. 42, 2306–2316. Leblanc, J.L., 2002. Rubber–filler interactions and rheological properties in filled compounds. Prog. Polym. Sci. 27, 627–687. McCabe, C.C., 1965. In: Kraus, G. (Ed.), Reinforcement of Elastomers, Vol. 7. Interscience, New York, pp. 225–268. Murphy, L.J., Wang, M.-J., Mahmud, K., 1998. Carbon–silica dual phase filler: Part III. ESCA and FTIR characterization. Rubber Chem. Technol. 71, 998–1014. Nakijama, N., Collins, E.A., 1975. Rheological properties of carbon black filled acrylonitrile-butadiene rubber, Paper presented at a meeting of Rubber Division, ACS. Osanaiye, G., Leonov, A.I., White, J.L., 1995. Rheological properties of rubber–carbon black and carbon black-oil compounds including measurements at very low shear rates. Rubber Chem. Technol. 68, 50–58. Patat, E., Killman, E., Schoietener, S., 1966. Adsorption of macromolecules from solution. Rubber Chem. Technol. 39, 36–87. Ray, S., Bhowmick, A.K., 2001. Novel electron beam modified surface coated silica fillers. Physical and chemical characteristics. J. Appl. Polym. Sci. 83, 2255–2268. Schaal, S., Coran, A.Y., 2000. The rheology and processability of tire compounds. Rubber Chem. Technol. 73, 225–239. Schaal, S., Coran, A.Y., Mowdood, S.K., 2000. The effect of certain recipe ingredients and mixing sequence on the rheology and processability of silica and carbon black filled tire compounds. Rubber Chem. Technol. 73, 240–252. Shanmugharaj, A.M., Bhowmick, A.K., 2002a. Modification of dual phase filler by electron beam irradiation: physical characterization. Rubber Chem. Technol. 75, 605–616. Shanmugharaj, A.M., Sabharwal, S., Majali, A., Tikku, V.K., Bhowmick, A.K., 2002b. Surface characterization of electron beam modified dual phase filler by ESCA, FT-IR and surface energy. J. Mater. Sci. 37, 2781–2793. Shanmugharaj, A.M., Bhowmick, A.K., 2003. Influence of novel electron beam modified surface treated dual phase filler on rheometric and mechanical properties of styrene– butadiene rubber vulcanizates, Rubber Chem. Technol. 76. Vinogradov, G.V., Ya Malkin, A., 1979. Rheology of Polymer. Mir Publishers, Moscow, p. 106, 304.
Radiation Physics and Chemistry 69 (2004) 91–98
Rheological properties of styrene–butadiene rubber filled with electron beam modified surface treated dual phase fillers A.M. Shanmugharaj, Anil K. Bhowmick* Rubber Technology Centre, Indian Institute of Technology, Kharagpur-721302 India Received 16 September 2002; accepted 21 March 2003
Abstract The rheological properties of styrene–butadiene rubber (SBR) loaded with dual phase filler were measured using Monsanto Processability Tester (MPT) at three different temperatures (100 C, 110 C and 130 C) and four different shear rates (61.3, 306.3, 613, and 1004.5 s1). The effect of electron beam modification of dual phase filler in absence and presence of trimethylol propane triacrylate (TMPTA) or triethoxysilylpropyltetrasulphide (Si-69) on melt flow properties of SBR was also studied. The viscosity of all the systems decreases with shear rate indicating their pseudoplastic or shear thinning nature. The higher shear viscosity for the SBR loaded with the electron beam modified filler is explained in terms of variation in structure of the filler upon electron beam irradiation. Die swell of the modified filler loaded SBR is slightly higher than that of the unmodified filler loaded rubber, which is explained by calculating normal stress difference for the systems. Activation energy of the modified filler loaded SBR systems is also slightly higher than that of the control filler loaded SBR system. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Styrene–butadiene rubber (SBR); Electron beam; Filler; Rheology; Die swell; Activation energy
1. Introduction The knowledge of rheological properties of elastomers is of considerable importance in predicting and comprehending their processing characteristics. Both the viscous and elastic properties of elastomer can be analyzed and correlated with the flow behavior. The viscous flow is related to the output rate, whereas the elastic behavior corresponds to the dimensional stability. Einhorn and Turetzky (1964) have shown the use of capillary rheometer to characterize elastomeric flow of carbon black filled styrene–butadiene rubber (SBR) systems. The processing behavior of carbon black filled elastomers and their blends have also been well *Corresponding author. Tel.: +91-3222-282037; fax: +913222-277190. E-mail address: [email protected] (A.K. Bhowmick).
explained in various literatures (Leblanc, 2002; Nakijama and Collins, 1975; Bhaumik et al., 1990; Kumar et al., 2002; Osanaiye et al., 1995). Cotton, 1979 has studied the post-extrusion relaxation phenomenon over a wide spectrum of carbon blacks using a capillary rheometer. Recently, the processing behavior of finely divided silica in combination with bifunctional silane has been reported (Schaal and Coran, 2000; Schaal et al., 2000). In these references, the rheological behavior of silica filled SBR has been explained as a function of filler type, filler loading, temperature, storage time and storage temperature. Recently, Cabot Corporation has designed new filler by co-fuming technology of carbon and silica, which is termed as carbon–silica dual phase filler having higher polymer–filler interaction and lower filler–filler interaction (Murphy et al., 1998). Even though, this new filler has lower amount of silica phase, its reinforcement behavior is close to silica. From our earlier communications (Shanmugharaj and Bhowmick, 2002a; Shanmugharaj
0969-806X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0969-806X(03)00331-1
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modified dual phase fillers in presence and absence of various sensitizers.
and Bhowmick, 2003; Shanmugharaj et al., 2002b), we have concluded that modification of carbon–silica dual phase filler by electron beam treatment results in surface oxidation and structure development that enhances certain properties like modulus, tensile strength and elongation at break. In this paper, we discuss the rheological behavior of SBR loaded with electron beam
2. Experimental 2.1. Materials Polymer: Styrene–butadiene rubber (SBR-1502, styrene content-23.5%, ML1+4 100 C, 51) was supplied by Synthetic and Chemicals Ltd, Barielley, India. Filler: Dual phase filler (CSDPF-A, Silica content 4.7%, N2 Surface area 167 m2/g) was supplied by Cabot Corporation, Billerica, USA. Trimethylol propane triacrylate: TMPTA was supplied by UCB Chemicals, Belgium. Silane coupling agent: Triethoxysilylpropyltetrasulphide (Si-69) was supplied by Degussa Company, Germany.
Table 1 Specification of the electron beam accelerator Energy range Beam power over the whole energy range Beam energy spread Average current (E1.5 MeV) Adjusting limits for current Accelerating voltage frequency Duration Repetition rate Pulse rate Maximum Minimum Power supply (PS) voltage PS voltage frequency Consumption of power (total)
0.5–2.0 MeV 20 kW 710% 15 mA 0–30 mA 100–120 MHz 400–700 ms 2–50 Hz
2.2. Filler modification The detailed procedures for the modification of filler using electron beam accelerator were discussed in our earlier communications (Shanmugharaj and Bhowmick, 2002a; Shanmugharaj et al., 2002b). The specifications of the electron beam accelerator are reported in Table 1 (Ray and Bhowmick, 2001).
900 mA 900 mA 3 380/200 V 50 Hz 150 kW
2.3. Preparation of filled SBR Table 2 Compounding formulation Sl. No
Ingredients
Loading (phr)
Mixing time (min)
1 2 3 4 5
SBR 1502 Filler ZnO Stearic acid Polymerized trimethyl quinoline
100.0 20.0 5.0 1.5 1.0
1 4 1 1 1
The compounds according to the formulation (Table 2) were mixed in a Brabender Plasticorder (model PLE 330, capacity 65 ml) at a rotor speed of 60 rpm and at 80 C. In all cases, mixing time was kept at 8 min. 2.4. Designation of filled SBR SBR masterbatch was designated as SBx/y/z, where S and B represent SBR and dual phase filler respectively, and the suffixes x; y and z represent monomer or silane, radiation dose (in kGy) and filler loading, respectively. The sample designations are tabulated in Table 3.
This filler master batch was passed in two-roll mill for 2 min at 30 C and used for rheological measurements.
Table 3 Sample designation for gum and filled SBR Sl. No
Sample designation
Filler
Modifier
% of modifiers
Radiation dose (kGy)
Filler loading (phr)
1 2 3 4 5 6 7 8
S0 (gum) SB0/0/20 SB0/100/20 SB0/200/20 SB1T/100/20 SB3T/100/20 SB1S/100/20 SB3S/100/20
— CSDPF-A CSDPF-A CSDPF-A CSDPF-A CSDPF-A CSDPF-A CSDPF-A
— — — — TMPTA TMPTA Si-69 Si-69
— — — — 1 3 1 3
— — — 100 200 100 100 100
— 20 20 20 20 20 20 20
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2.5. Measurement of rheological properties The melt flow properties of the gum SBR (without filler) and the unmodified and the electron beam modified filler loaded SBR were measured by means of a Monsanto Processibility Tester (MPT) (barrel radius, 9.53 mm) which is a fully automated capillary viscometer. The entire barrel and the capillary assembly are electrically heated with a microprocessor-based temperature control. The capillary used had a length to diameter ratio equal to 30 (length 30.00 mm; diameter 1.00 mm). The compound entrance angles of capillary were 45 and 60 , which are known to minimize the pressure drop at entrance. Therefore, the Bagley correction can be assumed to be negligible and the apparent shear stress can be taken as equal to the true shear stress. The preheated time for each sample was 5 min. The extrusion studies were carried out at three different temperatures (100 C, 110 C and 130 C) and at four different shear rates (61.3, 306.3, 613 and 1004.5 s1). The rate of shear variation was achieved by changing the speed of the plunger automatically. The pressure at the entrance of the capillary was recorded automatically with the help of pressure transducer. The apparent shear stress (tapp ), apparent shear rate (gapp ) and apparent shear viscosity (Zapp ) were calculated using the following equations (Brydson, 1981): tapp ¼
dc DP ; 4lc
ð1Þ
g’ app ¼
32Q ; pdc3
ð2Þ
Zapp ¼
tapp ; g’ app
ð3Þ
where DP is the pressure drop across the length of the capillary, dc and lc are diameter and length of capillary, respectively; Q; the volumetric flow rate of the material. The flow behavior index, n and consistency index, k were calculated by using the Power Law model tapp ¼ k’gapp
where C¼
3n0 þ 1 ; 4ðn0 þ 1Þ
ð8Þ
where a is the extrudate swell and n0 is equal to n, the flow behavior index determined from the slope of log (apparent shear viscosity) (Zapp ) vs. log (shear rate) (’gapp ) plots, following Eq. (6): 2.7. The principle normal stress differences The principle normal stress differences, sE ; in an elastic body, is calculated by using the following general equation (Vinogradov and Ya Malkin, 1979): sE ¼
ð2 þ gm Þgm ð2tÞ; 2ð1 þ gm Þ
ð9Þ
where t is the shear stress, gm the maximum recoverable deformation which is a complex function of filler loading, temperature and the nature of the fluid. 2.8. Activation energy of melt flow Activation energy of viscous flow was derived from the Arrhenious–Frenkel–Eyring (Vinogradov and Ya Malkin, 1979), which is valid for the power law fluids, as follows: Zapp ¼ BeEg =RT ;
ð10Þ
where, Eg is the activation energy of the flow at a particular shear rate, R; the gas constant, T; the temperature in Kelvin, B, the pre-exponential component and Zr ; the viscosity in Pa s at that shear rate. For the different systems, from the slope of the linear plot of log viscosity vs. reciprocal of temperature (1/T), the value of Eg =R was obtained from which the activation energy was calculated.
3. Results and discussion ð5Þ 3.1. Rheological behavior of electron beam modified filler loaded SBR
Logarithmic form for Eq. (5) may be written as log Zapp ¼ log k þ ðn 1Þlog g’ app :
ð7Þ
ð4Þ
by definition Zapp ¼ tapp =’gapp ; therefore Zapp ¼ k’gn1 app :
et al., 1992a): rffiffiffiffiffiffiffi 1 4 gm ¼ ða þ 2a2 3Þ; 2C
93
ð6Þ
The values n and k; were calculated for the initial linear region observed at lower shear rate. 2.6. Maximum recoverable deformation Maximum recoverable deformation, gm was calculated using the following equations (Kumar
Fig. 1 shows the apparent shear viscosity vs. shear rate plots for the gum and the unmodified and the electron beam modified dual phase filler loaded styrene– butadiene rubber at three different temperatures (100 C, 110 C and 130 C). The viscosity decreases with the shear rate showing the pseudoplastic or shear thinning nature of all the systems studied. Both the gum SBR and its filled compounds follow the power law model. The
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Fig. 1. Log (Apparent shear viscosity) vs. log (Apparent shear rate) plots of gum and electron beam modified filler loaded SBR at (a) 100 C (b) 110 C (c) 130 C.
Fig. 2. Log (Apparent shear viscosity) vs. log (Apparent shear rate) plots of TMPTA and silane modified filler loaded SBR at (a) 100 C (b) 110 C (c) 130 C.
viscosity of the gum SBR is lower than that of the filled systems at all the temperatures studied. As compared to the unmodified system (SB0/0/20), the modified filler loaded SBRs (SB0/100/20 and SB0/200/20) show slightly higher viscosity especially at lower shear rate region (Fig. 1). From TEM results (Shanmugharaj and Bhowmick, 2002a), it has been concluded that electron beam modification of dual phase filler results in variation of primary and secondary structures. It has been previously reported that anisometry of the filler aggregates and occluded rubber, which are associated with the structure of the filler may play an important role in compound viscosity (Patat et al., 1966; McCabe, 1965). The occlusion of the rubber within the aggregates enhances the effective hydrodynamic volume of the filler. In the case of modified filler loaded SBRs (SB0/100/20 and SB0/200/20), higher polymer occlusion takes place within the aggregates, which in turn enhances the effective hydrodynamic volume of the filler, giving rise to higher viscosity. Fig. 2 compares the viscosity vs. shear rate plots of TMPTA and silane modified dual
phase filler loaded SBR compounds (SB1T/100/20, SB3T/100/20, SB1S/100/20 and SB3S/100/20). The plots suggest that there is no significant variation in apparent viscosity for these modified filler loaded systems. In our earlier communications (Shanmugharaj and Bhowmick, 2002a) we have concluded that TMPTA or silane occupies the pores of the dual phase filler that significantly affects the rubber occlusion when this fillers are loaded in rubber. The consistency index, k and flow behavior index, n for this modified filler loaded SBR vulcanizates are included in Table 4. It is quite obvious that consistency index, k is more for the filled elastomer compared to the gum rubber. However, the consistency index, k is still higher for SB0/100/20 and SB0/200/20 compared to the control (SB0/0/20) showing the increase in resistance to flow. However, modification of the filler with TMPTA or with silane slightly improves the index value indicating the higher resistance to flow for this modified filler loaded SBR systems. The higher rubber occlusion due to its high structure and higher polymer–filler
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Table 4 Flow behavior index, n and consistency index k 104 (Pa sn) for SBR loaded with modified fillers Sl. No
Samples
Temperature 100 ( C)
1 2 3 4 5 6 7 8
S0 SB0/0/20 SB0/100/20 SB0/200/20 SB1T/100/20 SB3T/100/20 SB1S/100/20 SB3S/100/20
110 ( C)
130 ( C)
n
k ( 104 Pa sn)
n
k ( 104 Pa sn)
N
k ( 104 Pa sn)
0.40 0.38 0.38 0.35 0.39 0.37 0.35 0.35
37.33 48.08 48.98 66.83 48.53 52.36 58.62 60.30
0.41 0.44 0.43 0.41 0.39 0.42 0.42 0.50
30.40 33.82 42.39 49.85 42.98 48.12 42.10 48.10
0.42 0.49 0.51 0.42 0.42 0.41 0.41 0.43
27.30 30.43 34.91 35.89 36.39 39.90 33.42 39.80
Fig. 3. Variation of die swell with shear rate for gum and electron beam modified filler loaded SBR.
interactions due to chemical interaction in modified filler loaded systems (SB0/100/20, SB0/200/20, SB1T/100/20, SB3T/100/20, SB1S/100/20 and SB3S/100/20) results in higher consistency index for this modified filler loaded SBRs. Consistency index, k decreases with increase in temperature resulting in decrease in resistance to flow for the gum and the filled systems. The variation in flow behavior index, n; shows the pseudoplastic behavior of these SBR systems. 3.2. Elastic effects in melt flow 3.2.1. Effect of modified fillers on die swell of SBR Fig. 3 shows the variation of die swell with shear rate for the gum and the unmodified and the electron beam modified dual phase filler loaded SBR. Die swell remains almost constant upto a shear rate of about 306.3 s1 and then decreases at higher shear rate for the modified filler loaded systems. Die swell is low for the filled SBR compared to the gum rubber (S0 ), which is attributed to the polymer–filler interactions, reduction in polymer content per unit volume of the compound, decrease in elastic nature and development of higher viscosities. The
Fig. 4. Linear plots of log (normal stress difference) vs. log (shear stress) for gum and electron beam modified filler loaded SBR.
variation in die swell is almost same for all the filler loaded SBR at lower shear rate. However, at higher shear rate, the values of die swell are entirely different. SB0/100/20 and SB0/200/20 show higher values at higher shear rate (1004.5 s1) compared to SB0/0/20. In order to understand the elastic effects of this modified filler loaded SBR systems, principle normal stress difference is calculated at different shear rates using Eqs. (6)–(9). The logarithmic value of this normal stress difference when plotted against log (shear stress) gives a straight line as per the following equation (Kumar et al., 1992a) (Fig. 4): sE ¼ IðtÞm ;
ð11Þ
Log sE ¼ logðIÞ þ m logðtÞ:
ð12Þ
The slope and intercept values calculated by using the best-fit lines at two temperatures (110 C and 130 C) are included in Table 5. The slope value increases and the intercept value marginally decreases at both the temperatures (110 and 130 C) for electron beam modified dual phase filler loaded SBRs (SB0/100/20 and
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SB0/200/20) as compared to the control system (SB0/0/20). This is attributed to the decrease in elastic effect due to higher rubber occlusion, which enhances the viscous nature of the filled elastomer. The slope and intercept values of the double logarithmic plot of principle normal stress difference vs. shear stress (Table 5) confirms the above fact. Fig. 5 shows the plot of die swell vs. shear rate for TMPTA or silane modified dual phase filler loaded SBR. The die swell is almost same upto the shear rate of 306.3 s1 for TMPTA (SB1T/100/20 and SB3T/100/20) or silane (SB1S/100/20 and SB3S/100/20) modified filler loaded SBRs. However, it decreases with increase in silane level at higher shear rate (1004.5 s1). 3.2.2. Effect of temperature on die swell An increase in temperature reduces die swell of both the rubber and the filled systems (Fig. 6). The reduction in die swell is more in the case of gum than the filled systems. This can be well understood from the higher negative slope value (0.19) in the linear plot of die swell with temperature (Fig. 6a). The higher intercept Table 5 Slope and intercept values of gum and filled SBR calculated using Eq. (12) Sl. No Samples
Slope (m)
Intercept [log (I)]
at 110 C at 130 C at 110 C at 130 C 1 2 3 4 5 6 7 8
S0 SB0/0/20 SB0/100/20 SB0/200/20 SB1T/100/20 SB3T/100/20 SB1S/100/20 SB3S/100/20
0.95 0.97 1.03 1.05 0.98 1.05 1.04 1.05
0.89 0.91 0.94 0.97 0.92 0.93 0.91 0.93
2.00 1.96 1.84 1.78 1.85 1.81 1.86 1.81
2.15 2.10 2.07 1.93 2.07 2.04 2.12 2.00
Fig. 5. Variation of die swell with shear rate for TMPTA and silane modified filler loaded SBR.
Fig. 6. Variation of die swell with temperature for (a) gum and electron beam modified dual phase filler loaded SBR. (b) TMPTA and silane modified dual phase filler loaded SBR. Table 6 Slope and intercept values of the linear plot of die swell vs. temperature Sl. No
Samples
Slope
Intercept
1 2 3 4 5 6 7 8
S0 SB0/0/20 SB0/100/20 SB0/2200/20 SB1T/100/20 SB3T/100/20 SB1S/100/20 SB3S/100/20
0.19 0.17 0.11 0.08 0.06 0.12 0.10 0.12
81.20 67.90 64.50 62.50 58.06 65.86 64.00 65.66
value (81.20) for the gum rubber (Table 6) is attributed to higher rubber entanglements that store the elastic energy. The increase in temperature increases the mobility of the polymer chain, which reduces the capacity of polymer molecules to store the elastic energy and hence there is a reduction in die swell values (Kumar et al., 1992b). However, in the case of the filled rubber (SB0/0/20), the negative slope decreases to 0.17 and also the intercept value to 67.2. Addition of filler in rubber reduces the polymer content per unit volume, thereby decreasing the elastic nature. This is responsible for the lower intercept value for the filled elastomer. The lower negative slope for the filled elastomers is attributed to higher rubber occlusion in fillers that restricts the mobility of polymer chains on increasing the
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temperature. On incorporating the modified fillers in SBR (SB0/100/20 and SB0/200/20), the intercept and the negative slope values still decrease. This is also due to occluded rubber in the filler aggregates and rubber–filler interaction, which balance the effect of temperature. Presence of TMPTA or silane in the modified fillers reduces the effect of temperature on die swell for SBR composites (SB1T/100/20 and SB1S/100/20) compared to SB0/100/20 (Fig. 6b). This can be well documented from the slope and the intercept values (Table 6). However, on increasing the TMPTA or silane level in the filler, both the negative slope value and intercept value increases. Even though, rubber–filler interaction is more in SB3T/100/20 and in SB3S/100/20, rubber occlusion will be less due to the presence of TMPTA or silane in pores of filler. Hence, polymer per unit volume will be more in SB3T/100/20 and in SB3S/100/20 compared to SB1T/100/20 and SB1S/100/20 resulting in polymer flow with temperature.
3.3. Activation energy of melts flow Activation energy of the flow process has been calculated from the slope of the plot of log (shear viscosity) vs.1/T as given by Eq. (10) (Vinogradov and Ya Malkin, 1979). Fig. 7 shows the dependence of activation energy on the rate of shear for electron beam modified filler loaded SBR Activation energy of melt flow for the gum rubber (S0 ) is almost same for over the range of shear rates
97
investigated (Fig. 7a). SB0/0/20 and SB0/100/20 show almost constant value at lower shear rate (upto 306.3 s1) and decreases with further increase in shear rate. However, activation energy drastically reduces for SB0/200/20 even at lower shear rate (at 306.3 s1). This decrease in activation energy with increasing shear rate may be due to: (i) orientation of the molecular segments in the direction of applied stress and (ii) increased wall slip (Kumar et al., 1992b). Electron beam modification of filler results in variation of primary and secondary structures, which increases with radiation dose. The fractal dimensions as observed from TEM studies (Shanmugharaj and Bhowmick, 2002a) are 1.26 for the control filler (B0/0) and 1.36 for the electron beam modified filler (B0/100), respectively. This variation in structure results in more rubber occlusion that enhances the viscosity at lower shear rate, which is responsible for higher activation energy. However, at higher shear rates, breakdown of structures results in release of occluded rubber leading to wall slippage that decreases the activation energy of melt flow and this effect is more pronounced in SB0/200/20. The Eg value is low for SB0/0/20 and SB0/100/20, whereas it is high for SB0/200/20 compared to the gum rubber (S0 ) at higher shear rate (1004.5 s1). The lower value of Eg for SB0/0/20 and SB0/100/20 is attributed to the reduction in matrix viscosity due to decrease of rubber and broadening of molecular weight distribution (MWD) (Kumar et al., 1992a). The break down of structure at this shear rate releases rubber molecules with lower molecular weight thereby it increasing the molecular weight distribution, which decreases the matrix viscosity. Even though, structure breakdown is prominent in SB0/200/20, there may be significant amount of structure that enhances the matrix viscosity, which is responsible for the higher activation energy for SB0/200/20 compared to the gum (S0 ) at higher shear rate. Fig. 7b shows the variation of activation energy of melt flow with shear rate for TMPTA and silane modified dual phase filler loaded SBR. Activation energy decreases sharply with shear rate for all the modified filler loaded SBR systems. However, with increase in TMPTA or silane level, the activation energy of melt flow at lower shear rate region (61.3 s1) increases, which is attributed to the higher surface activity of the TMPTA and silane modified filler with the rubber matrix. However, at higher shear rate (1004.5 s1) the activation energy decreases with silane level due to dispersion of filler that reduces the matrix viscosity.
4. Conclusions Fig. 7. Variation of activation energy with shear rate for (a) gum and electron beam modified dual phase filler loaded SBR. (b) TMPTA and silane modified dual phase filler loaded SBR.
In the present investigation, the melt flow properties of the gum and the unmodified and the electron beam
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modified dual phase filler loaded SBR were measured and the following conclusions were drawn: 1. Apparent shear viscosity decreases with increasing shear rate for the gum and the unmodified and the electron beam modified dual phase filler loaded SBRs showing the pseudoplastic or shear thinning nature of the system. 2. The higher shear viscosity at all temperatures (100 C, 110 C and 130 C) for the modified filler loaded SBRs is attributed to higher rubber occlusion due to its high structure compared to the control dual phase filler loaded SBR. 3. Die Swell is almost same with increase in shear rate upto 306.3 s1 beyond which it decreases for the filler loaded SBR systems. This is explained by calculating normal stress difference for all the systems. 4. The activation energy of the melt flow process for filled SBR compounds decreases with increase in shear rate due to orientation of the molecular segments in the direction of applied stress and increased wall slippage.
Acknowledgements The authors are grateful to Dr. M. J. Wang, Cabot Corporation USA for providing the dual phase fillers. Dr. A. B. Majali & Dr. S. Sabharwal of Bhabha Atomic Research Centre, Mumbai for assistance in the electron beam work and Dr. V. K. Tikku of NICCO Corporation Ltd., Calcutta for his timely help.
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